1. Field of the Invention
The invention relates to a method and apparatus for straightening turbine casings.
2. Description of the Related Art
Through years of operation, turbine casings are repeatedly subjected to high thermally induced stresses above the yield point of the material. The reasons for this include the fact that outer surface of a inner casing of a steam turbine is normally cooled by exhaust steam, which is up to 300.degree. C. cooler than the highest steam temperature inside the casing. This leads to a significant steam temperature gradient in the wall of the inner casing. The temperature gradient over the casing wall causes thermal stress in the wall. Also, due to high thermal gradients which occur during start up, shut down, and abnormal operation, turbine casings distort during operation. The distortion is generally a collapse of both halves and get worse with time. In general, compressive stresses at the inner rim and tensile stresses at the outer rim are generated. In the case of shrink ring casing all stresses are compressive with higher stress at the inner rim.
Additionally, in the cold condition after operation, the negative creep strains introduced by the relaxation process during the operation period causes considerable tensile stress in the inner portion of the casing wall. Once the casing is disassembled and the casing halves can move freely, the bending portion of the stress disappears, thus causing a change of curvature of the casing wall and overall distortion of the casing.
Some of the stresses can become locked into the casing due to local yielding of adjacent material. This results in both casing distortion and residual casing stresses. The residual stresses continue to increase in size and quality resulting in cracking and/or casing distortion.
A distorted casing will also change the radial clearances to the rotor. Because a distorted casing takes the shape of an ellipse, the clearance between the rotor and casing will increase in some areas and will be reduced in others. Increased clearances provide efficiency loss, while decreased clearances increase the risk of rubbing and a possible forced outage. Providing a reliable method of rounding casings gives the industry a good incentive to perform rounding on a regular schedule to regain lost efficiency and to minimize the risk of a forced outage.
Distorted casings should be rerounded to allow trouble free assembly/disassembly of both halves, allow trouble free installation of blade carriers or diaphragms, close the horizontal joint face in order to minimize steam leakage, allow concentrical remachining, remove residual stresses, and establish design seal clearances.
A conventional method of straightening turbine casings is known as "hot spotting." This process includes local heating of the outside surface of the casing (spot heating). A small area is heated rapidly to introduce local yielding. The heating is done in small spot lines and covers the entire outer surface of the casing. In the area of the steam inlet pipes, the space is very limited and therefore no major rerounding can be achieved there. Stress relief of the casing may optionally be provided after the hot spotting. Otherwise, stress relief is performed after the rerounding operation.
Hot spotting is not an entirely satisfactory technique for straightening turbine casings. It can only be applied to rather flimsy casings. Also, several attempts need to be made and the results are not predictable. Spot heating by torch is uncontrolled and may overheat and alter the heated area of the casing. Features such as the steam inlet pipes do not allow the casing to straighten uniformly. Stress relief after straightening to reduce the possibility of cracking, also reduces the amount of rerounding achieved. The relaxation can only be estimated. Since the heat straightening provides a casing with a nonuniform collapse, matching of the seals and shrouds causes a problem. Seal clearances need to be increased depending on the remaining collapse and therefore the unit efficiency decreases.
Casing straightening using rounding plates has also been done for heavy wall casings with flanges, which cannot be straightened by heat spot straightening. The rounding plates are used during a stress relief cycle. The rounding plates are designed to be inserted into several areas of the casing to keep the casing in a defined shape during the stress relief process. Since the rounding plates are bigger then the casing, heat needs to be applied to the inside of the casing to open it and the process of installing the plates needs to be planned carefully.
The interior of the lower half of the casing is first heated with torches, while the opening of the casing is carefully monitored. Rounding plates are then installed as soon as the resulting expansion of the opening allows. After all rounding plates are installed in the lower casing, the upper casing is expanded in the same way. After verification that the applied heat has expanded the casing enough, it is laid onto the lower half.
Temporary bolts are then installed to clamp the two halves together. The casing with the rounding plates installed is then be stress relieved in an oven. Due to remaining residual stresses, the casing needs to be over-rounded. The "spring back" of the casing applies substantial force to the rounding plates, even after stress relief, and can make the separation of the two halves very tricky.
This method yields better results then the heat spot straightening and the outcome is more predictable. However it requires detailed planning and execution to insert the rounding plates, the potential exists of stocked rounding fixtures in the upper and/or lower half of the casing and the introduction of stresses to the casing in cold conditions.
See also, Herbert Barsch. A New Creep Equation for Ferritic and Martensitic Steels. Steel Research, No. 9, Vol. 66, 1995, p. 384ff.; On Site Reround of a Turbine Inner Cylinder by D. Ginn and R. Acton, ASME/IEEE Power Generation Conference, Milwaukee, Wis., Oct. 20-24, 1985; On-Site 430 MW High Pressure Turbine Shell Cracking and Distortion Repair by D. Rasmussen and M. Hilkey, all of which are hereby incorporated by reference.